1. Field of the Invention
This invention relates generally to storage and dispensing systems for the selective dispensing of gaseous reagents from a vessel in which the gas component(s) are held in sorptive relationship to a solid adsorbent medium, and are desorptively released from the sorbent medium in the dispensing operation.
2. Description of the Related Art
In the manufacture of semiconductor materials and devices, and in various other industrial processes and applications, there is a need for a reliable source of hydride and halide gases, as well as a wide variety of other process gases. Many such gases, including for example silane, germane, ammonia, phosphine, arsine, diborane, stibine, hydrogen sulfide, hydrogen selenide, hydrogen telluride, and corresponding and other halide (chlorine, bromine, iodine, and fluorine) compounds, as a result of toxicity and safety considerations, must be carefully stored and handled in the industrial process facility.
The gaseous hydrides arsine (AsH3) and phosphine (PH3) are commonly used as sources of arsenic (As) and phosphorous (P) in ion implantation. Due to their extreme toxicity and high vapor pressure, their use, transportation and storage raise significant safety concerns for the semiconductor industry. Ion implantation systems typically use dilute mixtures of AsH3 and PH3 at pressures as high as 1500 psig. A catastrophic release of gas from these high-pressure cylinders could pose a serious injury potential and even death to fab workers.
Based on these considerations, the ion implant user must choose between solid or gas sources for arsenic and phosphorous species. Switching from As to P on an implanter with solid sources can take as long as 90 minutes. The same species change requires only 15 minutes with gas sources. However, arsine (AsH3) and phosphine (PH3), the two most commonly used source gases, are highly toxic. Their use has recently been the focus of widespread attention due to the safety aspects of handling and processing these gases. Many ion implantation systems utilize hydride gas sources supplied as dilute mixtures (10-15%), in either 0.44 L or 2.3 L cylinders at pressures of 400-1800 psig. It is the concern over the pressure-driven release of the gases from cylinders that has prompted users to investigate safer alternatives.
U.S. Pat. No. 4,744,221 issued May 17, 1988 to Karl O. Knollmueller discloses a method of storing and subsequently delivering arsine, by contacting arsine at a temperature of from about xe2x88x9230xc2x0 C. to about +30xc2x0 C. with a zeolite of pore size in the range of from about 5 to about 15 Angstroms to adsorb arsine on the zeolite, and then dispensing the arsine by heating the zeolite to an elevated temperature of up to about 175xc2x0 C. for sufficient time to release the arsine from the zeolite material.
The method disclosed in the Knollmueller patent is disadvantageous in that it requires the provision of heating means for the zeolite material, which must be constructed and arranged to heat the zeolite to sufficient temperature to desorb the previously sorbed arsine from the zeolite in the desired quantity.
The use of a heating jacket or other means exterior to the vessel holding the arsine-bearing zeolite is problematic in that the vessel typically has a significant heat capacity, and therefore introduces a significant lag time to the dispensing operation. Further, heating of arsine causes it to decompose, resulting in the formation of hydrogen gas, which introduces an explosive hazard into the process system. Additionally, such thermally-mediated decomposition of arsine effects substantial increase in gas pressure in the process system, which may be extremely disadvantageous from the standpoint of system life and operating efficiency.
The provision of interiorly disposed heating coil or other heating elements in the zeolite bed itself is problematic since it is difficult with such means to uniformly heat the zeolite bed to achieve the desired uniformity of arsine gas release.
The use of heated carrier gas streams passed through the bed of zeolite in its containment vessel may overcome the foregoing deficiencies, but the temperatures necessary to achieve the heated carrier gas desorption of arsine may be undesirably high or otherwise unsuitable for the end use of the arsine gas, so that cooling or other treatment is required to condition the dispensed gas for ultimate use.
Despite the disadvantages of gaseous reagents at high pressures, high-pressure cylinder packaging continues to be pervasively used in the semiconductor manufacturing industry, for storage and delivery of a wide variety of semiconductor process gases. For example, boron trifluoride and silane are conventionally delivered to the semiconductor manufacturing facility and high-pressure gas cylinders. The gas storage capacity of the cylinders is usually determined and limited by the interior pressure of the gas cylinder. It therefore is advantageous, in increasing the storage capacity of the fixed-volume gas cylinders conventionally used in the industry, to increase cylinder pressures substantially. Unfortunately, however, this expedient increases the accompanying risks and hazards of leakage or catastrophic cylinder failure. Nonetheless, the pressurized gas cylinders conventionally employed in the industry have been in use for many years and are present in the market in very large numbers. Further, such pressurized gas cylinders must be approved for conventional transport by the U.S. Department of Transportation, and the appertaining DOT regulations are highly specific and widely accepted as governing the structure and operational characteristics of the gas package. The conventional high pressure gas cylinder therefore is firmly entrenched in the market, as a DOT-approved gas containment vessel.
Reducing the pressure level of gases in conventional gas cylinders, to enhance their safety, results in diminution of cylinder capacity, with associated reduced efficiency and productivity when such reduced pressure gas cylinders are employed as gas storage and dispensing vessels in a semiconductor manufacturing facility.
Gases in high-pressure gas cylinders as a result of their inherent dangers have an associated set of safety requirements. High-pressure gas cylinders are generally placed 200-250 feet from process tools, typically in dedicated gas rooms, in the semiconductor manufacturing facility. The high pressure level of such gas packaging also implicates the ambient environmental controls that must be practiced when high-pressure gas cylinders are placed in dedicated gas rooms or gas cabinets, since such rooms and cabinets must be ventilated at a rate determined by the pressure of the contained gases, so as to effectively remove any gas leakage from the cylinder into the ambient environment. In other words, a higher pressure gas cylinder will require a higher ventilating gas volumetric flow rate in order to xe2x80x9csweepxe2x80x9d the ambient environment of any gas leaking from the cylinder.
As a result of their toxicity and hazards, high-pressure gas cylinders have in various instances been replaced by liquid precursors in atmospheric pressure packages, which minimize release potentials of hazardous or toxic reagents.
The use of high-pressure gas cylinders with their associated elevated pressure levels also entails operational issues, including equipment and flow circuitry service lifetimes and maintenance intervals.
High-pressure gas cylinder usage entails reduction of lifetimes of valves, in comparison to valve operation at substantially lower pressures.
FIG. 1 shows the results of valve cycle testing, in the form of valve lifetime as a function of operating pressure, demonstrating this factor. Further, at reduced pressure, connections are easier to make leak-tight, and pressure surges are more readily reduced or eliminated. Valves at lower pressure seal more effectively and are less likely to leak and/or generate particles. Additionally, purge/vent cycles are more effective with less material to remove from process vessels and flow circuitry at lower pressures.
It therefore is apparent from the foregoing discussion that the conventional usage of high-pressure gas cylinders for storage and dispensing of gaseous reagents in semiconductor manufacturing operations, as well as in other industrial processes, has substantial associated disadvantages.
The present invention relates generally to storage and dispensing systems for the selective dispensing of gaseous reagents from a vessel in which the gas component(s) are held in sorptive relationship to a solid adsorbent medium, and are desorptively released from the sorbent medium in the dispensing operation.
In one specific aspect, the invention relates to a gas storage and dispensing system including a gas storage and dispensing vessel having an enclosed interior volume joined to gas dispensing means, wherein said interior volume contains a physical adsorbent and a gas in the interior volume and at least partially adsorbed on the physical adsorbent, wherein the gas pressure in said interior volume is at superatmospheric pressure, and wherein said gas pressure is lower than gas pressure of a same gas in a corresponding sorbent-free vessel and/or the amount of gas in said interior volume of the vessel is higher than the amount of the same gas in a corresponding sorbent-free vessel.
In another aspect, the invention relates to a method of increasing capacity of a high pressure gas cylinder for containing gas at a predetermined pressure level and/or decreasing the pressure of gas in said cylinder while maintaining at least as large an inventory of gas therein, said method comprising disposing in the interior volume of said vessel a physical adsorbent having sufficient sorptive affinity and capacity for said gas, so that the cylinder containing said physical adsorbent holds more gas at a corresponding pressure or has a gas pressure below said predetermined pressure level at a same gas inventory amount.
Other aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.